Polyimide / nano-Ti O2( PI/Ti O2) composite films were prepared via in-situ dispersive polymeriza-tion． The cross section surface morphology and composition of films were analyzed by SEM and XＲD． The purposein this paper is to research the effect of content on the electrical properties of PI / Ti O2composite films by electrical measure． The results show that inorganic Ti O2particles are homogeneously dispersed in the polymer matrices． Withthe Ti O2doping concentration increasing，breakdown voltage whose maximum is 240 k V / mm at 1% increases firstand then decreases; time-to-breakdown of the composite films can be lengthened continuously; dielectric constantwhose minimum and maximum are 3. 11(at 3% ) and 3. 49( at 7% ) decreases first and then increases． Conductivi-ty and dielectric loss nearly show no change after doping． In the frequency of 102Hz，the conductivities of compos-ite films are lower than 6. 0 × 1013S / cm． Nano-Ti O2doped composite films not only maintain excellent dielectricproperties of polyimide，but also possess outstanding heat stability．